Leads and lags between Antarctic temperature and carbon dioxide during the last deglaciation

To understand causal relationships in past climate variations, it is essential to have accurate chronologies of paleoclimate records. The last deglaciation, which occurred from 18 000 to 11 000 years ago, is especially interesting, since it is the most recent large climatic variation of global extent. Ice cores in Antarctica provide important paleoclimate proxies, such as regional temperature and global atmospheric CO2. However, temperature is recorded in the ice while CO2 is recorded in the enclosed air bubbles.The ages of the former and of the latter are different since air is trapped at 50–120 m below the surface. It is therefore necessary to correct for this air-ice shift to accurately infer the sequence of events.

Here we accurately determine the phasing between East Antarctic temperature and atmospheric CO2 variations during the last deglacial warming based on Antarctic ice core records. We build a stack of East Antarctic temperature variations by averaging the records from 4 ice cores (EPICA Dome C, Dome Fuji, EPICA Dronning Maud Land and Talos Dome), all accurately synchronized by volcanic event matching. We place this stack onto the WAIS Divide WD2014 age scale by synchronizing EPICA Dome C and WAIS Divide using volcanic event matching, which allows comparison with the high resolution CO2 record from WAIS Divide. Since WAIS Divide is a high accumulation site, its air age scale, which has previously been determined by firn modeling, is more robust. Finally, we assess the CO2/Antarctic temperature phasing by determining four periods when their trends change abruptly.

We find that at the onset of the last deglaciation and at the onset of the Antarctic Cold Reversal (ACR) period CO2 and Antarctic temperature are synchronous within a range of 210 years.Then CO2 slightly leads by 165 ± 116 years at the end of the Antarctic Cold Reversal (ACR) period.Finally, Antarctic temperature significantly leads by 406 ± 200 years at the onset of the Holocene period. Our results further support the hypothesis of no convective zone at EPICA Dome C during the last deglaciation and the use of nitrogen-15 to infer the height of the diffusive zone. Future climate and carbon cycle modeling works should take into account this robust phasing constraint.

Causal feedbacks in climate changeThe statistical association between temperature and greenhouse gases over glacial cycles is well documented1, but causality behind this correlation remains difficult to extract directly from the data. A time lag of CO2 behind Antarctic temperature—originally thought to hint at a driving role for temperature2, 3—is absent4, 5 at the last deglaciation, but recently confirmed at the last ice age inception6 and the end of the earlier termination II (ref. 7). We show that such variable time lags are typical for complex nonlinear systems such as the climate, prohibiting straightforward use of correlation lags to infer causation. However, an insight from dynamical systems theory8 now allows us to circumvent the classical challenges of unravelling causation from multivariate time series. We build on this insight to demonstrate directly from ice-core data that, over glacial–interglacial timescales, climate dynamics are largely driven by internal Earth system mechanisms, including a marked positive feedback effect from temperature variability on greenhouse-gas concentrations.

A release of carbon dioxide (CO2) from the deep ocean helped bring an end to the last Ice Age, according to new collaborative research by the University of Southampton, Universitat Autònoma de Barcelona (UAB), the Australian National University (ANU), and international colleagues.

Published today in Nature, the study shows that carbon stored in an isolated reservoir deep in the Southern Ocean re-connected with the atmosphere, driving a rise in atmospheric CO2 and an increase in global temperatures. The finding gives scientists an insight into how the ocean affects the carbon cycle and climate change. Atmospheric CO2 levels fluctuate from about 185 parts-per-million (ppm), during ice ages, to around 280 ppm, during warmer periods like today (termed interglacials). The oceans currently contain approximately sixty times more carbon than the atmosphere and that carbon can exchange rapidly (from a geological perspective) between these two systems (atmosphere-ocean).

Joint lead author Dr. Miguel Martínez-Botí from the University of Southampton adds: “The magnitude and rapidity of the swings in atmospheric CO2 across the ice age cycles suggests that changes in ocean carbon storage are important drivers of natural atmospheric CO2 variations. Joint lead author Dr. Gianluca Marino, from ANU and previously at the ICTA, UAB, says: “We found that very high concentrations of dissolved CO2 in surface waters of the Southern Atlantic Ocean and the eastern equatorial Pacific coincided with the rises in atmospheric CO2 at the end of the last ice age, suggesting that these regions acted as sources of CO2 to the atmosphere”. “Our findings support the theory that a series of processes operating in the southernmost sector of the Atlantic, Pacific and Indian Oceans, a region known as the ‘Southern Ocean’, changed the amount of carbon stored in the deep-sea. While a reduction in communication between the deep-sea and the atmosphere in this region potentially locks carbon away from the atmosphere into the abyss during ice ages, the opposite occurs during warm interglacial periods.”

The international team studied the composition of the calcium carbonate shells of ancient marine organisms that inhabited the surface of the ocean thousands of years ago in order to trace its carbon content. Co-author Dr. Gavin Foster from the University of Southampton commented: “Just like the way the oceans have stored around 30 per cent of humanity’s fossil fuel emissions over the last 100 years or so, our new data confirms that natural variations in atmospheric CO2 between ice ages and warm interglacials are driven largely by changes in the amount of carbon stored in our oceans. “These results will help to better understanding the dynamics of human-induced CO2 accumulation in the atmosphere since the ocean is an important carbon sink and the largest reservoir of carbon on our planet’ commented co-author Patrizia Ziveri, ICREA professor at the ICTA, UAB. While these new results support a primary role for the Southern Ocean processes in these natural cycles, we don’t yet know the full story and other processes operating in other parts of the ocean, such as the North Pacific, may have an additional role to play.

In a paper published today in Science, researchers from the University of Bristol describe how they used radiocarbon measured in deep-sea fossil corals to shed light on carbon dioxide (CO2) levels during the Earth’s last deglaciation.

Around 18,000-11,000 years ago, the Earth’s climate system experienced a dramatic shift: a period known to paleoclimate scientists as the last deglaciation. During this period, atmospheric CO2 concentration increased by ~80 parts per million (ppm), accompanied by sea level rise of almost 120 metres due to ice sheet melting and global warming. Recent high-resolution ice core CO2 records have revealed that there were three abrupt centennial-scale atmospheric CO2 increases of ~10 ppm superimposed on the more gradual millennial-scale deglacial CO2 rise. The second and third of these events also coincided with abrupt warming of the high latitude North Atlantic region.

The rate of Atlantic Meridional Overturning Circulation – that is, the deep water formation in the high latitudes and associated upwelling – is closely related to the temperature of the North Atlantic region and thus might also be related to these CO2 releasing events. However it has been remarkably hard to find marine archives that can show how deep oceans behave on rapid timescales. Researchers from the University of Bristol, University of St Andrews and University of California Irvine tackled this problem using radiocarbon measured in deep-sea fossil corals. The corals were recovered by scientific research expeditions to the Equatorial Atlantic and Southern Ocean, funded by the European Research Council and the US National Science Foundation.

Fossil corals have the unique advantage that they can be precisely dated by radiometric uranium-series dating, giving an age scale that can be directly compared to the ice core records. Radiocarbon is introduced into the ocean at the surface and penetrates to deeper layers through deep water formation. During this process radiocarbon decays away, so that deep-sea radiocarbon – and, therefore, the reconstructed fossil coral radiocarbon – can provide information on the past strength of deep ocean circulation. The measurements revealed two massive transient events where the water becomes homogenized and enriched in radiocarbon in the mid-depth equatorial Atlantic and the Drake Passage, in phase with the second two abrupt increases of the atmosphere CO2 concentration during the last deglaciation. Lead author, Dr Tianyu Chen of Bristol’s School of Earth Sciences said: “Our radiocarbon data are consistent with two transient and enhanced deep Atlantic overturning events which flushed out respired carbon in the deep water, causing a rapid rise of atmosphere CO2 concentration and abrupt warming of the high latitude North Atlantic.”

Climate is not constant on Earth. Consider ice ages coming and going as an example. Parallel to ice age cycles, atmospheric carbon dioxide reduces during glacial periods and increases during warmer times, although modern fossil fuel-related carbon dioxide emission broke this natural cyclicity. With the proper proxy measurements, scientists can look into these past cycles to determine how exactly climate systems were naturally governed.Syracuse University Earth sciences Assistant Professor Zunli Lu says, “A million dollar question in understanding climate system is: Where and how was CO2 sequestered from the atmosphere during ice ages?”

Lu and international collaborators explored the question of carbon dioxide storage in the oceans. The team glimpsed into the ocean’s past, thanks to a group of tiny ocean dwellers called foraminifera. The foraminifera were preserved in sediment cores taken from an underwater mountain in the Amundsen Sea, which is part of the ocean surrounding Antarctica. Their study on the ocean records, trapped in foraminifera shells, is published in Nature Communications. Although it is accepted that atmospheric carbon dioxide was sucked deep into the ocean during ice ages, quantitative measures of how much was stored, and in what chemical form, have been hard to come by. One way to track carbon dioxide storage is by investigating its partner in respiration: oxygen. Plankton photosynthesize near the ocean surface, taking in carbon dioxide. When the plankton die and sink to the ocean floor, the reverse process, respiration, happens. Respiration uses up oceanic oxygen and re-releases carbon dioxide in the deep ocean. Because of this relationship, low oxygen levels in a specific part of the ocean can flag where atmospheric carbon dioxide was stored during glacial periods.

While past oceanic oxygen levels can provide insight to global carbon dioxide cycling, reconstructing past oxygenation conditions is quite a challenge. Enter foraminifera. Foraminifera have an external shell made of calcium carbonate, which traps signatures of their environment as they grow–including oxygen levels. These climate-related signatures are often preserved for millions of years in sediments accumulated on the ocean floor. Lu has spent years measuring iodine in calcium carbonate as a proxy for tracking oceanic oxygen levels. “These carbonates recorded almost the entire Earth’s history , and we have one of the best ways of translating the oxygen stories out of these rocks and fossils” he says. Unlike most other oxygen proxies, iodine is able to detect relatively subtle changes in oxygen levels, not just presence or absence of the gas in seawaters. Study co-author Babette Hoogakker of the University of Oxford seconds the power of this new proxy: “Its application allows the assessment of open ocean subsurface oxygenation states using planktonic foraminifera, which until now was virtually impossible.”

Foraminifera from the group’s study site in the high-latitude Antarctic indicate that the surrounding Southern Ocean reached very low levels of oceanic oxygen during ice ages. Co-author Ros Rickaby, also of Oxford, says, “It was a real surprise to discover that any part of the Southern Ocean, which today is so rich in oxygen, evolved naturally to contain such small amounts of this influential gas during the glacial period.” Co-author Claus-Dieter Hillenbrand of the British Antarctic Survey says, “Physical, chemical and biological changes in the World Ocean in general, and in the Southern Ocean in particular, played a crucial role for glacial-interglacial variations in atmospheric carbon dioxide concentrations and global climate change.”

Or as Lu puts it: “The Southern Ocean is like a tap for CO2. When you open the tap, CO2 goes into the atmosphere and it becomes warm globally. When you partially shut that valve during an ice age, CO2 becomes lower in the atmosphere.” Lu says that finding low oxygen in the high-latitude Southern Ocean gives insight into how the “valve” works during ice ages, and moreover points to the oxygenation conditions in vast volumes of deep waters connected to the Southern Ocean through ocean circulation. Lu and colleagues hope that understanding the past cycles will give researchers foresight into the effects of modern and future global changes. While the level of atmospheric carbon dioxide routinely cycled between 200 and 280 parts per million in the past, the planet is currently faced with levels of 400 parts per million of the gas. “If we can explain the naturally oscillating recent climate, we may be able to make predictions for how modern warming might cause the marine environment to change in the future,” Lu says.